The present invention relates generally to color images and more particularly to methods and apparatus for identifying the colors of an image.
The color of every element in an image can be identified by three values, generally known as its XYZ values. Each value corresponds to the spectral information in one spectral band, and the three values together identify the color of an element in the image. The XYZ values depend on the responses of the human eyes and are instrument independent. The values are known as CIE standard responsivities. A general discussion of such standards can be found in "Color Science: Concepts and Methods, Quantitative Data And Formulae," second edition, written by Wyszecki and Stiles, and published by John Wiley and Sons.
Numerous electronic instruments have been used to identify colors of an image. The instruments typically measure the color of each element and identify it by three values, generally known as its RGB or Red-Green-Blue values. The color of each element of the image is identified by measuring the element with three sensors, with each sensor corresponding to one of the three components, red, green or blue. One such instrument is a scanner.
Different instruments have different sensors with different characteristics, which produce different RGB values. Therefore, the RGB values are instrument dependent. It would be very beneficial to transform the instrument dependent RGB values to the instrument independent XYZ values to simplify the hardware and software manipulating the values.
If the RGB values can be transformed to the XYZ values by a three by three matrix with minimal errors, then such a transformation is known as a linear transformation. Instruments capable of performing linear transformation are known as colorimetric instruments.
Prior art inexpensive scanners are quite noncolorimetric. For each element, the calculated or scanner-identified XYZ values are significantly different from the actual XYZ values. Moreover, the errors introduced by the transformation process depend on the media, such as the paper and ink, used to produce the image.
One way to improve on the transformation linearity is to use more sensors to measure each element. Typical scanners employ arrays of charge coupled devices (CCD) as sensors. These CCD arrays are expensive. Using more CCD devices to reduce the errors in the transformation process is a very expensive approach.
There is still a need to improve on the instrument and the method to linearly transform the RGB values to the XYZ values. Further, it would be desirable if the improvement does not incur significant cost.